Normally, when a person fixes their attention on an object and follows it with his/her gaze, both eyes move in unison keeping them focussed on the same point. But, if certain pathologies are present in the subject, this does not happen and some people, largely children, suffer a deviation of one eye with respect to another, so that whilst one eye remains fixed looking at a point (fixating eye), the other does not do so, instead it remains in a deviated position (deviated or strabismic eye). This ophthalmologic disease is often known with the name of strabismus.
There are many ways of dividing the classes of strabismus; one of the most important is that it depends on the variability of the eye deviation:                Concomitant: the deviation of the eyes does not vary or varies little in the different gaze positions. This is the most frequent strabismus in a child.        Incomitant: the deviation of the eyes varies according to the gaze positions or is only manifested in some of them. This group includes the strabismus produced by paralysis of the nerves which are directed towards the muscles or by lesions of the muscles (degeneration or citatrization).        
Furthermore, depending on the direction of the eye deviation, strabismus can be classified as:                Esotropia (inward)        Exotropia (outward)        Hypertropia (upward)        Hypotropia (downward)        Cyclotropia (inward or outward rotation)        
From an epidemiological perspective, although strabismus was a pathology that saw its frequency in developed countries decrease from the decade of the seventies until the end of the century, due to the narrowing in the population base, this trend has reversed in recent years due to different factors present in current society:                1. The new increase in birth rate        2. Greater susceptibility of suffering the disease in new rising population groups: immigrants, adopted children from less developed countries and premature babies with the increase in assisted fertilization techniques (in vitro, etc.).        
Therefore, it is estimated that the incidence of strabismus in developed countries is approximately 4%, which means an increase in children undergoing said ophthalmologic diagnosis and treated for this type of pathology could be expected during one or two decades.
Specifically, in the province of Barcelona around 5000 ocular motility examinations are carried out on children annually, also exceeding one thousand examinations in adults, which gives rise to a high number of strabismus surgical procedures and more particularly, in the Hospital Sant Joan of Déu, around 500 children are operated on for strabismus per year.
Taking the overall statistics into consideration, the need arises for a constant adaptation of ophthalmologic medicine to the populations' changing health requirements, placing emphasis on the first phases of the disease and the prevention with diagnostic systems better adapted to patients and which cover the greatest possible number of pathologies in its detection functionality, which can be extended to the collaboration in treatment and monitoring of eye disease.
To date, a multitude of systems are known which permit determining and measuring possible ocular mobility alterations, but they all have one or several of the following problems                Active collaboration is required from the patient, which is difficult to achieve for children under the age of 6 and especially in mentally handicapped patients.        The lack of precision in measuring the ocular parameters leads to the excessively subjective interpretation of the results obtained with most of the existing diagnosis systems.        The strabismus detecting means known do not permit measuring deviation of the eyes at all points in the field of ocular motility of a person.        
In this regard, we can quote International Patent application PCT WO 93/19661 which provides an apparatus for testing ocular motility, which incorporates a light projector controlled by a computer and a screen where a light spot is projected, which the subject undergoing said eye examination should follow with his/her gaze. Therefore, the patient should be sitting down staring at the screen, if he/she is a child, there is usually little collaboration and means of fixing the child's head or controlling his/her movements needs to be included in the system to be able to obtain valid eye motility tests with the method set down in WO 93/19661. Furthermore, this method and the apparatus that executes it are based on the use of a pair of video cameras disposed frontally, in the visual axis of each eye respectively, but their position does not permit recording their entire visual field, and, in particular, prevents recording a downward gaze, since the upper lid covers the eye. Another deficiency is that the video cameras are fixed only recording the eyes, but means for capturing movements of the head are not provided, which means the ocular motility detected with this method and apparatus is not correct if the patient, as is typical in children or the mentally handicapped, moves his/her head.
Another example of apparatus to evaluate the alignment of the eyes is that disclosed in U.S. Pat. No. 5,094,521, which involves a drastic solution to the problem posed if the patient makes voluntary or involuntary head movements, avoiding this by immobilizing the head with tapes which are fixed to a support wherein the person should place his/her face, with the chin resting on a chinrest, so that the eyes remain perfectly aligned, through which he/she is made to look at a screen with light spots. Evidently, its seems very improbable that a young person or people with mental problems would tolerate the rigid fixing of his/her head, which in any case is uncomfortable even simply using velcro tapes. Furthermore, the fixed screen, which is positioned in front of the subject and has multiple light spots, incorporates a video camera in the centre to record both eyes at the same time. Nor can the camera, therefore, as in the previous apparatus, of document WO 93/19661, capture the movement of the eyes when the patient looks down, being very sensitive to small head movements which, even with the fixing tapes, the person makes on rotating the eyes in a sideward gaze, which falsifies the evaluation of ocular deviation.
An alternative which resolves the possible head movements, voluntary or involuntary, connected to eye movement, is the unit disclosed in European Patent EP 0940117, where the patient is free to move his/head, since it has means of control which detects its position, calculating the coordinates on the three axes (x, and, z) of the head by infrared. Infrared is also used to measure ocular motility, having an infrared detector for each eye, but these detectors do not move together with the head, but the captured measurements of the eye movements are independent from those taken concerning whether the patient moves his/her head. Children or even adults with any mental or physical disorders which causes chaotic movements of the head may cause too large movements of their head position, it being practically impossible to make reliable calculations of the eye movements, with respect to the point at which the gaze is directed, as it is easily possible to go outside the capturing area marked by the infrared detecting means. For these and other motives, such as the fact that the patient should be in continuous communication with the examiner by means of a helmet with microphone and headphones, the functioning of this system entails its exclusive application to very collaborative adults for it to be really effective, at least in the conceptual aspect.